Glass compositions compatible with downdraw processing and methods of making and using thereof

ABSTRACT

Described herein are alkali-free, boroalumino silicate glasses exhibiting desirable physical and chemical properties for use as substrates in flat panel display devices, such as, active matrix liquid crystal displays (AMLCDs). The glass compositions possess numerous properties that are compatible with the downdraw process, particularly fusion drawing.

BACKGROUND

The production of liquid crystal displays such as, for example, activematrix liquid crystal display devices (AMLCDs) is very complex, and theproperties of the substrate glass are extremely important. First andforemost, the glass substrates used in the production of AMLCD devicesneed to have their physical dimensions tightly controlled. The downdrawsheet drawing processes and, in particular, the fusion process describedin U.S. Pat. Nos. 3,338,696 and 3,682,609, both to Dockerty, are capableof producing glass sheets that can be used as substrates withoutrequiring costly post-forming finishing operations such as lapping andpolishing. Unfortunately, the fusion process places rather severerestrictions on the glass properties, which require relatively highliquidus viscosities.

In the liquid crystal display field, thin film transistors (TFTs) basedon poly-crystalline silicon are preferred because of their ability totransport electrons more effectively. Poly-crystalline based silicontransistors (p-Si) are characterized as having a higher mobility thanthose based on amorphous-silicon based transistors (a-Si). This allowsthe manufacture of smaller and faster transistors, which ultimatelyproduces brighter and faster displays.

One problem with p-Si based transistors is that their manufacturerequires higher process temperatures than those employed in themanufacture of a-Si transistors. These temperatures range from 450° C.to 600° C. compared to the 350° C. peak temperatures employed in themanufacture of a-Si transistors. At these temperatures, there areseveral properties of the glass composition that need to be taken intoconsideration. For example, the coefficient of thermal expansion (CTE)should be in a range such that there is minimal distortion to silicontransistors during cool-down from the high-temperature annealing step.It is desirable to minimize the CTE of the glass composition. Otherproperties of the glass composition to consider include the density andYoung's modulus, which contribute to the propensity of the glass sheetto sag. The sheet geometry is dictated by the particular processemployed, which is beyond the control of the glass manufacturer. Forfixed density, an increase in Young's modulus is desirable since itreduces the amount of sag exhibited by large glass sheets duringshipping, handling and thermal processing. Likewise, any increase indensity should be accompanied by a proportionate increase in Young'smodulus or it will result in increased sag. Thus, glass compositionswith a low CTE and high specific modulus (i.e, low density and highYoung's modulus) are desirable.

Other properties of the glass composition if not in the proper range canadversely affect the glass-making process. For example, if the glass has200 poise temperature that is very high, this creates a problem forpremelt refractories and possible erosion of Pt/Rh in the finer. If theglass has a high stir, the formation of Pt inclusions in the glass arepossible. Additionally, if the delivery temperature of the glass ishigh, this can present a problem for isopipe corrosion and sag. Finally,the use of chemical fining agents are needed to control liquidustemperature, liquidus viscosity and liquidus phase (cristobalite) forthe fusion draw process. However, chemical fining agents are limited intheir ability to control these properties. Moreover, chemical finingagents such as, for example, arsenic, are generally not preferred in theglass-making process due to environmental concerns.

Described herein are alkali-free glasses and methods for making the samethat possess a number of desirable properties required for downdrawprocessing, which is important in the manufacturing of substrates forliquid crystal displays.

SUMMARY

In accordance with the purposes of the disclosed materials, compounds,compositions, articles, devices, and methods, as embodied and broadlydescribed herein are alkali-free, boroalumino silicate glassesexhibiting desirable physical and chemical properties for use assubstrates in flat panel display devices, such as, active matrix liquidcrystal displays (AMLCDs). The glass compositions possess numerousproperties that are compatible with the downdraw processing. Additionaladvantages will be set forth in part in the description that follows,and in part will be obvious from the description, or may be learned bypractice of the aspects described below. The advantages described belowwill be realized and attained by means of the elements and combinationsparticularly pointed out in the appended claims. It is to be understoodthat both the foregoing general description and the following detaileddescription are exemplary and explanatory only and are not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, which are incorporated in and constitute apart of this specification, illustrate several aspects described below.

FIG. 1 is a plot of liquidus viscosity vs. liquidus temperature minussoftening point for a wide range of LCD-type glasses.

DETAILED DESCRIPTION

The materials, compounds, compositions, articles, devices, and methodsdescribed herein may be understood more readily by reference to thefollowing detailed description of specific aspects of the disclosedsubject matter and the Examples included therein and to the Figures.

Before the present materials, compounds, compositions, articles,devices, and methods are disclosed and described, it is to be understoodthat the aspects described below are not limited to specific syntheticmethods or specific reagents, as such may, of course, vary. It is alsoto be understood that the terminology used herein is for the purpose ofdescribing particular aspects only and is not intended to be limiting.

Also, throughout this specification, various publications arereferenced. The disclosures of these publications in their entiretiesare hereby incorporated by reference into this application in order tomore fully describe the state of the art to which the disclosed matterpertains. The references disclosed are also individually andspecifically incorporated by reference herein for the material containedin them that is discussed in the sentence in which the reference isrelied upon.

Throughout the description and claims of this specification the word“comprise” and other forms of the word, such as “comprising” and“comprises,” means including but not limited to, and is not intended toexclude, for example, other additives, components, integers, or steps.

As used in the description and the appended claims, the singular forms“a,” “an,” and “the” include plural referents unless the context clearlydictates otherwise. Thus, for example, reference to “a composition”includes mixtures of two or more such compositions, reference to “anagent” includes mixtures of two or more such agents, reference to “thelayer” includes mixtures of two or more such layers, and the like.“Optional” or “optionally” means that the subsequently described eventor circumstance can or cannot occur, and that the description includesinstances where the event or circumstance occurs and instances where itdoes not.

Certain materials, compounds, compositions, and components disclosedherein can be obtained commercially or readily synthesized usingtechniques generally known to those of skill in the art. For example,the starting materials and reagents used in preparing the disclosedcompounds and compositions are either available from commercialsuppliers or prepared by methods known to those skilled in the art.

Reference will now be made in detail to specific aspects of thedisclosed materials, compounds, compositions, articles, and methods,examples of which are illustrated in the accompanying Examples andFigures.

Described herein are alkali-free glasses and methods for making the samethat possess properties compatible with downdraw (e.g., fusion)processes.

In one aspect the composition comprises an alkali-free glass comprisingin mole percent on an oxide basis:

SiO₂ 67.0-70.0  B₂O₃ 8.0-11.0 Al₂O₃ 9.5-12.0 MgO <3.7 CaO 5.5-11.2 SrO≦2.2 BaO ≦2.2 MgO/CaO ≦0.7

wherein:

-   (a) 1.0≦Σ(MgO+CaO+SrO+BaO)/(Al₂O₃)≦1.25, where Al₂O₃, MgO, CaO, SrO,    and BaO represent the mole percents of the representative oxide    components;-   (b) 2.5 MgO+5 CaO+6 SrO+7 BaO≦59;-   (c) the glass has a coefficient of thermal expansion less than or    equal to 36×10⁻⁷/° C. over the temperature range of 0 to 300° C.;-   (d) the glass has a density less than or equal to 2.46 g/cc; and-   (e) the glass has a liquidus temperature minus softening point up to    200° C.

To be formed by a downdraw process, in particular the fusion process,the temperature at which crystals first appear in the molten glass ispreferably maintained at as high a viscosity as reasonably possible. Inone aspect, the viscosity is at least 85 kilopoise (Kpoise), or greaterthan 100 Kpoise. It has been discovered that the difference between thegradient boat liquidus and softening point is an accurate gauge ofliquidus viscosity for AMLCD-type glasses. For example, FIG. 1 is a plotof liquidus viscosity vs. liquidus temperature minus softening point fora wide range of LCD-type glasses, which shows that when liquidustemperature minus softening point is less than or equal to 200° C., theliquidus viscosity is greater than 100 Kpoise. Therefore, in one aspect,a compatible glass for sheet formation via a downdraw process has aliquidus temperature minus softening point not greater than 200° C., orfrom 90° C. to 200° C. In another aspect, the glass composition has aliquidus temperature less than or equal to 1,200° C., or from 1,060° C.to 1,200° C. As shown in FIG. 1, lower values of liquidus temperatureminus softening point correspond to higher liquidus viscosities. Thus,keeping liquidus temperature minus softening point as low as reasonablypossible facilitates the production of glass sheets useful in LCDapplications.

Two additional features the glass compositions described herein possessinclude relatively low coefficients of thermal expansion (CTE) andelevated specific moduli. In one aspect, the CTE is less than or equalto 36×10⁻⁷/° C. over the temperature range of 0 to 300° C. In anotheraspect, the CTE is 30×10⁻¹⁷/C≦CTE≦36×10⁻⁷/° C. over the temperaturerange of 0 to 300° C.

During the processing of displays, glass sheets are often held only atopposite edges, and therefore experience sagging in the unsupportedcentral portion of the sheet. The amount of sag is a function of thegeometry of the sheet, the density, and Young's modulus of the glass,which together can be expressed as the specific modulus. The sheetgeometry is dictated by the particular process employed, which is beyondthe control of the glass manufacturer. For fixed density, an increase inYoung's modulus is desirable since it reduces the amount of sagexhibited by large glass sheets during shipping, handling and thermalprocessing. Likewise, any increase in density should be accompanied by aproportionate increase in Young's modulus or it will result in increasedsag. In one aspect, the glass has a density of less than or equal to2.46 grams/cc. In another aspect, the glass has a density from 2.37 g/ccto 2.46 grams/cc. In a further aspect, the glass has a Young's modulusfrom 10 Mpsi to 11.0 Mpsi, or from 10.4 Mpsi to 10.8 Mpsi.

The glass compositions described herein also possess high strain points.A high strain point is desirable to help prevent panel distortion due tocompaction/shrinkage during thermal processing subsequent tomanufacturing of the glass. In one aspect, the glass compositionsdescribed herein have a strain point greater than or equal to 650° C. orfrom 650° C. to 695° C. In a further aspect, the glass compositionsdescribed herein have a thermal compaction less than 30 ppm, less than25 ppm, less than 20 ppm, less than 15 ppm, or less than 10 ppm.

Each of the components used to prepare the glass compositions describedherein is discussed next. In the glass compositions, SiO₂ serves as thebasic glass former. In certain aspects, the concentration of SiO₂ can begreater than 67 mole percent in order to provide the glass with adensity and chemical durability suitable for a flat panel display glass(e.g., an AMLCD glass), and a liquidus temperature (liquidus viscosity),which allows the glass to be formed by a downdraw process (e.g., afusion process). In terms of an upper limit, in general, the SiO₂concentration is be less than or equal to about 70 mole percent to allowbatch materials to be melted using conventional, high volume, meltingtechniques, e.g., Joule melting in a refractory melter. As theconcentration of SiO₂ increases, the 200 poise temperature (meltingtemperature) generally rises. In various applications, the SiO₂concentration is adjusted so that the glass composition has a meltingtemperature less than or equal to 1,650° C.

Al₂O₃ is another glass former used to make the glass composition. Itsconcentration is determined by the total concentration of SiO₂+B₂O₃ andthe desired RO/Al₂O₃ ratio. In practical terms, the Al₂O₃ concentrationof the glass is from about 9.5 to about 12 mole percent. Such levels arerequired to provide adequate viscosity at the liquidus temperature toobtain a liquidus viscosity compatible with fusion. The use of at least9.5 mole percent Al₂O₃ also improves the glass' strain point andmodulus. In order to achieve an Σ[RO]/[Al₂O₃] ratio greater than orequal to 1, the Al₂O₃ concentration is kept below 12.0 mole percent, orbetween 9.5 and 11.5 mole percent.

B₂O₃ is both a glass former and a flux that aids melting and lowers themelting temperature. To achieve these effects, the glass compositionshave B₂O₃ concentrations that are equal to or greater than 8.0 molepercent. Large amounts of B₂O₃, however, lead to reductions in strainpoint (approximately 14° C. for each mole percent increase in B₂O₃ above8 mole percent), modulus, and chemical durability. In one aspect, B₂O₃concentration is between 8.0 and 11.0 mole percent, or between 8.5 and10.5 mole percent.

The Al₂O₃ and B₂O₃ concentrations can be manipulated to achieve thedesired strain point, modulus, durability, density, and CTE whilemaintaining the melting and forming properties of the glass. Forexample, an increase in B₂O₃ and a corresponding decrease in Al₂O₃ canlower the density and CTE, while an increase in Al₂O₃ and acorresponding decrease in B₂O₃ can increase strain point, modulus, anddurability, provided that the increase in Al₂O₃ does not reduce theΣ[RO]/[Al₂O₃] ratio below 1.

In addition to the glass formers (SiO₂, Al₂O₃, and B₂O₃), the glasscompositions also include at least two alkaline earth oxides. In oneaspect, the at least two alkaline earth oxides include MgO and CaO, and,optionally, SrO and/or BaO. The alkaline earth oxides provide the glasswith various properties important to melting, fining, forming, andultimate use. The MgO concentration in the glass and the glass'Σ[RO]/[Al₂O₃] ratio, where [Al₂O₃] is the mole percent of Al₂O₃ andΣ[RO] equals the sum of the mole percents of MgO, CaO, SrO, and BaO, canstrongly influence glass performance (e.g., meltability and fining). Anincrease in MgO concentration also increases the RO/Al₂O₃ ratio greaterthan 1.0, which contributes to the production of bubble-free glass. Inone aspect, the RO/Al₂O₃ ratio is greater than or equal to one and lessthan or equal to 1.25. Relative to the other alkaline earth oxides, thepresence of MgO results in lower density and CTE, and a higher chemicaldurability, strain point, and modulus. In one aspect, the MgOconcentration is greater than or equal to 1.0 mole percent and less than3.7 mole percent.

Of the alkaline earth oxides, the CaO concentration in the glasscomposition is generally the highest. The presence of CaO produces lowliquidus temperatures (high liquidus viscosities), high strain pointsand moduli, and CTEs in the most desired ranges for flat panelapplications (e.g., AMLCD applications). Compared to similar levels ofSrO or BaO, CaO also contributes favorably to chemical durability and isrelatively inexpensive as a batch material. In one aspect, The CaOconcentration is greater than or equal to 5.5 mole percent to achievethese desirable traits. At high concentrations, however, CaO increasesdensity and CTE relative to the major glass forming oxides and MgO.Therefore, in one aspect, the CaO concentration of the glasses of theinvention is less than or equal to 11.2 mole percent.

MgO and CaO work together to produce ideal physical and rheologicproperties for alkali-free AMLCD substrates. In certain aspects, oncethe MgO concentration is high enough, and the CaO concentration lowenough, mullite appears as a liquidus phase, and liquidus temperaturesincrease very rapidly with further increases in MgO concentration.Therefore, in one aspect, the molar ratio of MgO/CaO is less than orequal to 0.7 to avoid mullite as a liquidus phase.

The remaining alkaline earth oxides SrO and BaO can both contribute tolow liquidus temperatures (high liquidus viscosities). Thus, in certainaspects, the glass compositions will contain at least one of theseoxides. Relative to MgO or CaO, both SrO and BaO increase CTE anddensity and lower the modulus and strain point. Of the two, BaOgenerally has more adverse effects on glass properties than SrO, but hasa much greater impact on liquidus temperature, and hence on liquidusviscosity. In one aspect, SrO and BaO can be present to a level of up to2.2 mole percent without altering the physical properties outside of thedesired ranges discussed above. In another aspect, both SrO and BaO canbe present at a combined level of up to 2.7 mole percent withoutcompromising physical properties and manufacturing characteristics.These elevated concentrations can aid in obtaining a sufficiently highliquidus viscosity so that the glass can be formed by a downdrawprocess.

The interplay between the alkaline earth oxides produces very complexliquidus and physical property dependencies. In one aspect, when B₂O₃,SiO₂ and RO/Al₂O₃ are within the ranges described above, the amount ofalkaline earth oxides can be expressed by the following:

2.5MgO+5CaO+6SrO+7BaO≦59

where the oxides are in mole percent. This weighting is in accordancewith the impact of these oxides on physical properties. As an example,BaO is given a high weight because it contributes greatly to CTE anddensity, while the low weight for MgO reflects is comparatively benignor advantageous impact on these attributes.

In addition to the above components, the glass compositions describedherein can include other oxides to adjust various physical, melting,fining, and forming attributes of the glasses. Examples of such otheroxides include, but are not limited to, TiO₂, MnO, ZnO, Nb₂O₅, MoO₃,Ta₂O₅, WO₃, Y₂O₃, La₂O₃, CeO₂, or any combination thereof. In oneaspect, the amount of each of these oxides is less than or equal to 2.0mole percent, and their total combined concentration is less than orequal to 5.0 mole percent. The glass compositions can also includevarious contaminants associated with batch materials and/or introducedinto the glass by the melting, fining, and/or forming equipment used toproduce the glass, such as, Fe₂O₃ and ZrO₂. The glasses can also containSnO₂ either as a result of Joule melting using tin-oxide electrodesand/or through the batching of tin containing materials, e.g., SnO₂,SnO, SnCO₃, SnC₂O₄, etc.

The glass compositions are generally alkali free; however, the glass canalso contain some alkali contaminants. However, for AMLCD applications,the alkali levels should be kept below 0.1 mole percent to avoid havinga negative impact on thin film transistor (TFT) performance throughdiffusion of alkali ions from the glass into the silicon of the TFT. Asused herein, an “alkali-free glass” is a glass having a total alkaliconcentration that is less than or equal to 0.1 mole percent, where thetotal alkali concentration is the sum of the Na₂O, K₂O, and Li₂Oconcentrations. Preferably, the total alkali concentration is less thanor equal to 0.07 mole percent.

As discussed above, in accordance with the invention, it has been foundthat having an Σ[RO]/[Al₂O₃] ratio greater than or equal to one improvesfining (i.e., the removal of gaseous inclusions from the melted batchmaterials). This improvement allows for the use of more environmentallyfriendly fining packages. For example, on an oxide basis, the glasscompositions can have one or more of the following compositionalcharacteristics:

(i) an As₂O₃ concentration of at most 0.05 mole percent;

(ii) an Sb₂O₃ concentration of at most 0.05 mole percent; and/or

(iii) a SnO₂ concentration of at least 0.01 mole percent.

As₂O₃ is the most effective high-temperature fining agent for AMLCDglasses, and in certain aspects, As₂O₃ may be used for fining because ofits superior fining properties. In addition, if a melting systemincludes a region in which glass is in direct contact with platinum oralloys of platinum, and if the interface on the outside of the platinumcomprises air or moist air, then hydrogen can pass through the platinumat high temperature to the air interface, leaving behind an oxygen-richbubble. As₂O₃ is exceptionally good at consuming the oxygen in thebubble more or less at the rate that it is generated, therebyeliminating gaseous defects from the final glass. However, As₂O₃ ispoisonous and thus requires special handling during the glassmanufacturing process. When added to a base glass, As₂O₃ typicallyincreases liquidus temperature, and therefore its use to control defectsmust be balanced against its impact on liquidus viscosity. Accordingly,in one aspect, fining is performed without the use of substantialamounts of As₂O₃, i.e., the finished glass has at most 0.05 mole percentAs₂O₃. Preferably, no As₂O₃ is intentionally added to the glass. In thisaspect, the finished glass will typically have at most 0.005 molepercent As₂O₃ as a result of contaminants present in the batch materialsand/or the equipment used to melt the batch materials.

Sb₂O₃ contributes to fining and to suppression of oxygen-rich bubbles atglass-Pt interfaces. It is considerably less effective than arsenic inboth regards. Thus, higher concentrations are typically required toobtain the same impact as As₂O₃. Although not as toxic as As₂O₃, Sb₂O₃is also poisonous and requires special handling. In addition, Sb₂O₃raises the density, raises the CTE, and lowers the strain point incomparison to glasses that use As₂O₃ or SnO₂ as a fining agent. ForSb₂O₃ to be used at an appreciable concentration in a given composition,the BaO concentration is reduced by one mole percent per mole percentSb₂O₃, or the SrO concentration is reduced by 1.5 mole percent per molepercent Sb₂O₃, in order to keep physical properties within the rangesdescribed above. The impact of Sb₂O₃ on viscoelastic properties issimilar to B₂O₃ in that it lowers viscosity at all temperatures.However, Sb₂O₃ can decrease the stability of aluminosilicate crystals,and thereby extend the cristobalite liquidus to lower SiO₂ contents.This can be a desirable outcome for improving liquidus viscosity. IfSb₂O₃ is to be used as a fining agent or as a means to suppress blisterformation in the platinum system, then its concentration is preferablykept below 0.4 mole percent, more preferably at or below 0.3 molepercent.

In certain preferred embodiments, fining is performed without the use ofsubstantial amounts of Sb₂O₃, i.e., the finished glass has at most 0.05mole percent Sb₂O₃. Preferably, no Sb₂O₃ is purposely used in the finingof the glass. In such cases, the finished glass will typically have atmost 0.005 mole percent Sb₂O₃ as a result of contaminants present in thebatch materials and/or the equipment used to melt the batch materials.

Compared to As₂O₃ and Sb₂O₃ fining, tin fining (i.e., SnO₂ fining) isstill less effective, and its ability to absorb oxygen produced byhydrogen permeation is still more circumscribed; however, SnO₂ is aubiquitous material which has no known hazardous properties.Additionally, for many years, SnO₂ has been a component of AMLCD glassesthrough the use of tin oxide electrodes in the Joule melting of thebatch materials for such glasses. The presence of SnO₂ in AMLCD glasseshas not resulted in any known adverse effects when the glass is used tomanufacture liquid crystal displays. SnO₂, however, can form crystallinedefects in AMLCD glasses when used at high concentrations. Accordingly,the concentration of SnO₂ in the finished glass is preferably less thanor equal to 0.15 mole percent.

Tin fining can be used alone or in combination with other finingtechniques if desired. For example, tin fining can be combined withhalide fining, e.g., bromine fining. Other possible combinationsinclude, but are not limited to, tin fining plus sulfate, sulfide,cerium oxide, mechanical bubbling, and/or vacuum fining. In otheraspects, these other fining techniques can be used alone or in anycombination without the use of tin fining. Likewise, addition of SnO₂may permit reduction of As₂O₃ and/or Sb₂O₃, thus resulting in a moreenvironmentally friendly glass. In all of these cases, maintaining anΣ[RO]/[Al₂O₃] ratio, MgO concentration, and MgO/CaO ratios within theranges discussed above makes the fining process easier to perform andmore effective.

The glasses described herein can be manufactured using varioustechniques known in the art. In one aspect, the glasses are made using adowndraw process such as, for example, a fusion downdraw process. In oneaspect, described herein is a method for producing an alkali-free glasssheet by a downdraw process comprising selecting, melting, and finingbatch materials so that the glass making up the sheets comprises SiO₂,Al₂O₃, B₂O₃, MgO, CaO and BaO, and, on an oxide basis, comprises:

SiO₂ 67.0-70.0  B₂O₃ 8.0-11.0 Al₂O₃ 9.5-12.0 MgO <3.7 CaO 5.5-11.2 SrO≦2.2 BaO ≦2.2 MgO/CaO ≦0.7wherein:

-   (a) 1.0≦Σ(MgO+CaO+SrO+BaO)/(Al₂O₃)≦1.25, where Al₂O₃, MgO, CaO, SrO,    and BaO represent the mole percents of the representative oxide    components;-   (b) 2.5 MgO+5 CaO+6 SrO+7 BaO≦59; and-   (c) the fining is performed without the use of substantial amounts    of either arsenic or antimony; and-   (d) a population of 50 sequential glass sheets produced by the    downdraw process from the melted and fined batch materials    comprising an average gaseous inclusion level of less than 0.10    gaseous inclusions/cubic centimeter, where each sheet in the    population has a volume of at least 500 cubic centimeters.

In one aspect, the population of 50 sequential glass sheets produced bythe downdraw process from the melted and fined batch materials has anaverage gaseous inclusion level of less than 0.05 gaseousinclusions/cubic centimeter, where each sheet in the population has avolume of at least 500 cubic centimeters.

The downdraw sheet drawing processes and, in particular, the fusionprocess described in U.S. Pat. Nos. 3,338,696 and 3,682,609 both toDockerty, which are incorporated by reference, can be used herein.Compared to other forming processes, such as the float process, thefusion process is preferred for several reasons. First, glass substratesmade from the fusion process do not require polishing. Current glasssubstrate polishing is capable of producing glass substrates having anaverage surface roughness greater than about 0.5 nm (Ra), as measured byatomic force microscopy. The glass substrates produced by the fusionprocess have an average surface roughness as measured by atomic forcemicroscopy of less than 0.5 nm. The substrates also have an averageinternal stress as measured by optical retardation which is less than orequal to 150 psi. The glass compositions described herein can be used tomake various glass articles. For example, the glass compositionsdescribed herein can be used to make substrates for liquid crystaldisplays such as, for example, AMLCDs.

EXAMPLES

The following examples are set forth below to illustrate the methods andresults according to the disclosed subject matter. These examples arenot intended to be inclusive of all aspects of the subject matterdisclosed herein, but rather to illustrate representative methods andresults. These examples are not intended to exclude equivalents andvariations of the present invention which are apparent to one skilled inthe art.

Efforts have been made to ensure accuracy with respect to numbers (e.g.,amounts, temperature, etc.) but some errors and deviations should beaccounted for. Unless indicated otherwise, parts are parts by weight,temperature is in ° C. or is at ambient temperature, and pressure is ator near atmospheric. There are numerous variations and combinations ofreaction conditions, e.g., component concentrations, temperatures,pressures and other reaction ranges and conditions that can be used tooptimize the product purity and yield obtained from the describedprocess. Only reasonable and routine experimentation will be required tooptimize such process conditions.

Example 1 Preparation of a Test Sample

Test glass samples are made by melting appropriate batch materials in Ptcrucibles at 1,600-1,650° C. for 6 or more hours, and pouring onto asteel sheet, followed by annealing at approximately 720° C. The glassproperties in Table 1 were determined in accordance with techniquesconventional in the glass art. Thus, the linear coefficient of thermalexpansion (CTE) over the temperature range 0-300° C. (expressed in termsof ×10⁻⁷/° C.) and the strain point (expressed in terms of ° C.) weredetermined from fiber elongation techniques (ASTM references E228-85 andC336, respectively). The density in terms of grams/cm³ was measured viathe Archimedes method (ASTM C693). When reported, the meltingtemperature in terms of ° C. (defined as the temperature at which theglass melt demonstrates a viscosity of 200 poises) was calculatedemploying a Fulcher equation fit to high temperature viscosity datameasured via rotating cylinders viscometry (ASTM C965-81). The liquidustemperature of the glass in terms of ° C. was measured using thestandard gradient boat liquidus method of ASTM C829-81. This involvesplacing crushed glass particles in a platinum boat, placing the boat ina furnace having a region of gradient temperatures, heating the boat inan appropriate temperature region for 24 hours, and determining by meansof microscopic examination the highest temperature at which crystalsappear in the interior of the glass. The liquidus viscosity in poiseswas determined from the liquidus temperature and the coefficients of theFulcher equation. Young's modulus values in terms of Mpsi weredetermined using a resonant ultrasonic spectroscopy technique of thegeneral type set forth in ASTM E1875-00e1.

Throughout this application, various publications are referenced. Thedisclosures of these publications in their entireties are herebyincorporated by reference into this application in order to more fullydescribe the compounds, compositions and methods described herein.

Various modifications and variations can be made to the materials,methods, and articles described herein. Other aspects of the materials,methods, and articles described herein will be apparent fromconsideration of the specification and practice of the materials,methods, and articles disclosed herein. It is intended that thespecification and examples be considered as exemplary.

1. An alkali-free glass comprising in mole percent on an oxide basis:SiO₂ 67.0-70.0  B₂O₃ 8.0-11.0 Al₂O₃ 9.5-12.0 MgO <3.7 CaO 5.5-11.2 SrO≦2.2 BaO ≦2.2 MgO/CaO ≦0.7

wherein: (a) 1.0≦Σ(MgO+CaO+SrO+BaO)/(Al₂O₃)≦1.25, where Al₂O₃, MgO, CaO,SrO, and BaO represent the mole percents of the representative oxidecomponents; (b) 2.5 MgO+5 CaO+6 SrO+7 BaO≦59; (c) the glass has acoefficient of thermal expansion less than or equal to 36×10⁻⁷/° C. overthe temperature range of 0 to 300° C.; (d) the glass has a density lessthan or equal to 2.46 g/cc; and (e) the glass has a liquidus temperatureminus softening point up to 200° C.
 2. The glass of claim 1, wherein theglass has a linear coefficient of thermal expansion (CTE) of 30×10⁻⁷/°C.≦CTE≦36×10⁻⁷/° C. over the temperature range of 0 to 300° C.
 3. Theglass of claim 1, wherein the glass has a density of from 2.37 g/cc to2.46 g/cc.
 4. The glass of claim 1, wherein the glass has a liquidustemperature minus softening point from 90° C. to 200° C.
 5. The glass ofclaim 1, wherein the glass has a strain point greater than 650° C. 6.The glass of claim 1, wherein the glass has a strain point from 650° C.to 695° C.
 7. The glass of claim 1, wherein the glass has a Young'smodulus from 10 Mpsi to 11.0 Mpsi.
 8. The glass of claim 1, wherein theglass has a Young's modulus from 10.4 Mpsi to 10.8 Mpsi.
 9. The glass ofclaim 1, wherein the glass has a liquidus temperature from 1,060° C. to1,200° C.
 10. The glass of claim 1, wherein the glass has a viscosity atthe liquidus temperature minus softening point greater than 100 kpoise.11. The glass of claim 1, wherein the amount of B₂O₃ present in theglass composition is from 8.5 to 10.5 mole percent.
 12. The glass ofclaim 1, wherein the amount of Al₂O₃ present in the glass composition isfrom 9.5 to 11.5 mole percent.
 13. The glass of claim 1, wherein theamount of MgO present in the glass composition is from 1.0 to 3.7 molepercent.
 14. The glass of claim 1, wherein the glass is substantiallyfree of As₂O₃ and Sb₂O₃.
 15. The glass of claim 1, wherein the glasscomprises at least one of the following compositional characteristics:(a) at most 0.05 mole percent As₂O₃ on an oxide basis; (b) at most 0.05mole percent Sb₂O₃ on an oxide basis; or (c) at least 0.01 mole percentSnO₂ on an oxide basis.
 16. The glass of claim 1, wherein the glasscomprises a dimensional change less than 30 ppm.
 17. The glass of claim1, where in the glass further comprises TiO₂, MnO, ZnO, Nb₂O₅, MoO₃,Ta₂O₅, WO₃, Y₂O₃, La₂O₃, CeO₂, or any combination thereof, wherein theamount of each oxide is less than or equal to 2.0 mole percent.
 18. Theglass of claim 1, wherein the glass comprises the following properties:(a) the glass has a coefficient of thermal expansion from 30 to36×10⁻⁷/° C. over the temperature range of 0 to 300° C.; (b) the glasshas a strain point from 650° C. to 695° C.; (c) the glass has a densityof from 2.37 g/cc to 2.46 g/cc; (d) the glass has a Young's modulus from10.4 Mpsi to 10.8 Mpsi; (e) the glass has a liquidus temperature from1,060° C. to 1,200° C.; and (f) the glass has a liquidus temperatureminus softening point from 90° C. to 200° C.
 19. A method for producingan alkali-free glass sheet by a downdraw process comprising selecting,melting, and fining batch materials so that the glass making up thesheets comprises SiO₂, Al₂O₃, B₂O₃, MgO, CaO and BaO, and, on an oxidebasis, comprises: SiO₂ 67.0-70.0  B₂O₃ 8.0-11.0 Al₂O₃ 9.5-12.0 MgO <3.7CaO 5.5-11.2 SrO ≦2.2 BaO ≦2.2 MgO/CaO ≦0.7

wherein: (a) 1.0Σ(MgO+CaO+SrO+BaO)/(Al₂O₃)≦1.25, where Al₂O₃, MgO, CaO,SrO, and BaO represent the mole percents of the representative oxidecomponents; (b) 2.5 MgO+5 CaO+6 SrO+7 BaO≦59; and (c) the fining isperformed without the use of substantial amounts of either arsenic orantimony; and (d) a population of 50 sequential glass sheets produced bythe downdraw process from the melted and fined batch materialscomprising an average gaseous inclusion level of less than 0.10 gaseousinclusions/cubic centimeter, where each sheet in the population has avolume of at least 500 cubic centimeters.
 20. The method of claim 19,wherein the population of 50 sequential glass sheets produced by thedowndraw process from the melted and fined batch materials has anaverage gaseous inclusion level of less than 0.05 gaseousinclusions/cubic centimeter, where each sheet in the population has avolume of at least 500 cubic centimeters.
 21. The method of claim 19,wherein the glass sheet comprises one or more of the followingproperties: (a) the glass has a coefficient of thermal expansion from 30to 36×10⁻⁷/° C. over the temperature range of 0 to 300° C.; (b) theglass has a strain point from 650° C. to 695° C.; (c) the glass has adensity of from 2.37 g/cc to 2.46 g/cc; (d) the glass has a Young'smodulus from 10.4 Mpsi to 10.8 Mpsi; (e) the glass has a liquidustemperature from 1,060° C. to 1,200° C.; and (f) the glass has aliquidus temperature minus softening point from 90° C. to 200° C. 22.The method of claim 19, wherein the glass sheet comprises at least oneof the following compositional characteristics: (a) at most 0.05 molepercent As₂O₃ on an oxide basis; (b) at most 0.05 mole percent Sb₂O₃ onan oxide basis; or (c) at least 0.01 mole percent SnO₂ on an oxidebasis.
 23. The method of claim 19, wherein the glass sheet issubstantially free of As₂O₃, Sb₂O₃, and SnO₂.
 24. The method of claim19, wherein the glass sheet comprises a dimensional change less than 30ppm.
 25. The method of claim 19, wherein the downdraw process comprisesa fusion draw process.
 26. The glass sheet produced by the method ofclaim
 19. 27. A liquid crystal display substrate comprising the glasscomposition of claim 1.